Integration of chemical precipitation and nanofiltration for sodium sulfate control and brine loop closure in a Chlor Alkali plant

In membrane-based chlor-alkali processes, brine recirculation is essential for improving operational sustainability; however, sodium sulfate (Na 2 SO 4 ) accumulation in the loop hinders chlorine production. This study evaluates an integrated treatment scheme combining chemical precipitation and nanofiltration (NF) to remove Na 2 SO 4 and enhance effluent reuse in a chlor-alkali plant in Guayaquil, Ecuador. Laboratory precipitation tests using real industrial effluent examined the influence of calcium chloride (CaCl 2 ) dosage, temperature, and pH. The best removal (49.3 ± 4.2% w/w) was achieved with 45 mL L -1 CaCl 2 at 70 °C, pH 9-11, and 20 min of mixing, while efficiencies fell below 30% at 40-55°C even with higher dosages (≥ 50 mL L -1 ). Full-scale NF trials showed 72.8% w/w Na 2 SO 4 rejection at 25 bar with 67.9% w/w water recovery. Both treated effluents were suitable for reintegration into the brine circuit. Mass balance analysis indicated that the combined process recovers up to 81.3% of the effluent, equivalent to 8,051 m 3 year -1 of water and 1,393 t year -1 of NaCl, while mitigating saline discharges. The estimated capital cost (11,7021 USD) is offset by annual raw material savings of 93,964 USD. Overall, integrating chemical precipitation with membrane separation enhances process circularity, reduces freshwater consumption, and mitigates environmental impacts, while ensuring economic viability. • Factorial design to maximize impurity removal, analyzing T, dose, pH, and agitation • Optimized CaCl 2 dosing achieved up to 49.3% sulfate removal from real effluents • NF and chemical precipitation enables 81.3% effluent reuse in chlor-alkali plant • Process reduces saline discharge and recovers 8051 m 3 y -1 water and 1393 t y -1 NaCl • Recovered water and salt translate into a 9-year payback period